OBJECTIVE Low vitamin D levels predict the development of diabetes. This double-blind, randomized, control study in subjects with prediabetes and hypovitaminosis D evaluated whether high doses of vitamin D for 1 year affected insulin secretion, insulin sensitivity, and the development of diabetes.

RESEARCH DESIGN AND METHODS A total of 1,551 subjects ?40 years of age not known to have diabetes were screened with A1C levels. Subjects with A1C levels of 5.8–6.9% underwent an oral glucose tolerance test (OGTT). Subjects with prediabetes and 25-OH vitamin D (25-OHD) levels &lt;30 ng/mL were randomized to receive weekly placebo (N = 53) or vitamin D (N = 56) with doses based on body weight and baseline 25-OHD levels. OGTTs were performed 3, 6, 9, and 12 months later. Insulin secretion and sensitivity were measured, and the proportion of subjects developing diabetes was assessed.

RESULTS 25-OHD levels rapidly rose from 22 to nearly 70 ng/mL after vitamin D supplementation with a mean weekly dose of 88,865 IU. There were no differences between the placebo and vitamin D groups regarding

CONCLUSIONS In individuals with prediabetes and hypovitaminosis D, doses of vitamin D supplementation designed to raise serum 25-OHD levels into the upper-normal range for 1 year had no effect on insulin secretion, insulin sensitivity, or the development of diabetes compared with placebo administration.